The sentence that most clearly combines the sentences: "Sedentary lifestyles are harmful. Sedentary lifestyles result in disease;" into one cohesive thought is the last option listed. "Sedentary lifestyles are harmful and result in disease."
A cohesive thought in this case is a unit consisting of parts that fit together well and as a result form a united whole. In this case, AND serves to join both parts to convey the right meaning of the two different sentences.
